1. Field of the Invention
The present invention relates to a method and apparatus for converting methanol to gasoline (MTG). More particularly, the present invention relates to an improved method and apparatus for dewatering the feed to an MTG conversion reactor.
2. Discussion of the Prior Art
There are presently known numerous catalytic processes for the conversion of methanol to gasoline boiling range components by contacting methanol with ZSM-5 type zeolite catalyst. During the conversion process water, in the form of steam, is formed as a by-product and tends to deactivate the ZSM-5 catalyst. This steam deactivation is permanent and catalyst activity cannot be restored by regeneration. Catalyst life is influenced very strongly by the steam partial pressure and the temperature at which the reactor is operating. The amount of steam in the reactor effluent also serves to dictate the maximum temperature of the reactor operation. Consequently, the presence of water in the conversion reactor affects both the life of the ZSM-5 type catalyst and the temperature at which the reactor operates. The temperature of the reactor will influence to a certain extent the products formed and an undesirable product, durene, increases as a percentage of the output when the reactor temperature is lowered. If the durene yield could be reduced to a 2 wt.% level or lower, then dealkylation or heavy gasoline treatment (HGT) sometimes used in MTG systems can be eliminated and the product distillation unit can be simplified.
Methanol may be converted to gasoline in a two-stage operation. First, methanol is partially dehydrated, approaching an equilibrium mixture of dimethylether (DME), methanol and water in a conventional dehydration reactor. The methanol and DME are then supplied to the conversion reactor and are converted to hydrocarbon and water, which are subsequently separated out. Thus, there are three sources of water in the feed to the conversion reactor. First, there is generally 4-17 wt.% and sometimes 20 wt.% of water in the crude methanol feed, depending upon the methanol synthesis process employed upstream of the MTG system. Second, additional water is formed in the DME reactor because 2 moles of methanol converted to DME yields 1 additional mole of water. The dehydration reactor has an equilibrium range of 75-80%, and thus a large portion of water is created from the dehydration reaction. Third, water is carried by fuel gas, some of which is recycled to the conversion reactor.
FIG. 1 illustrates a conventional methanol-to-gasoline (MTG) converter in which crude methanol is supplied to dehydration reactor 10 and provides an output, which is a mixture of DME, methanol and water. The output mixture is then supplied to the conversion reactor 12, wherein it reacts over a ZSM-5 type catalyst in a known manner. The output of the conversion reactor will comprise a mixture of Liquified Petroleum Gas (LPG) and gasoline boiling range hydrocarbons, fuel gas and water which can be separated in product separator 14. A major portion of the fuel gas is recycled to the input of the conversion reactor, to control reactor temperature rise and the remainder comprising a fuel gas product.
Initially it was thought that with the FIG. 1 type MTG systems, crude methanol containing up to about 20 wt.% water content would be a satisfactory feedstock. Thus, no attempt was made to reduce water content in such a commercial MTG plant design. Although non-water removal is suitable for low water containing coal-based methanol feeds, it has been found that for other feeds it is desirable to reduce the water content of the methanol to 4 wt.% or lower. FIG. 2 illustrates the system of FIG. 1, in which a high water content crude methanol (17 wt.% H.sub.2 0) undergoes distillation at generally atmospheric pressure through a large diameter tower, with the distilled methanol (4 wt.% or less H.sub.2 O) being supplied as the input to dehydration reactor 10. This provides a minor reduction in the water content of the conversion reactor. However, as can be seen by a simple example, the water content of the crude methanol input varies only to a minor degree the water content in the feed to the conversion reactor. If a natural gas-based crude methanol with 17 wt.% water is utilized, the water in the feed to the conversion reactor is about 3.8 wt.%. For a coal-based crude methanol feed which has a 4 wt.% water content and the same recycle gas ratio, the water in the reactor feed is about 3.0 wt.%. Thus, reducing the crude methanol water content from 17 to 4 wt.% only reduces the water content of the reactor feed from 3.8 to 3.0 wt.%. After reaction in the conversion reactor, the water in the conversion reactor effluent is 7.1 and 6.4 wt.% for 17 and 4 wt.% water feeds, respectively. It would be desirable to reduce as much as possible the water content in the conversion reactor feed itself, which would serve to control the first two of the three sources of water in the reactor feed. The third source is fixed by the equilibrium condition in the product separator and is not readily controllable.
U.S. Pat. No. 4,210,495 describes a process for producing a stream of purified methanol which includes feeding a water-methanol mixture containing small quantities of ethanol, ketones, and higher alcohols to a first distillation column. Product methanol is withdrawn from the upper level of the first column, and a sidestream of aqueous methanol is directed from the first column to a second distillation column. A second product stream of methanol is obtained from the upper level of the second column. The first distillation column is operated at low pressure (6.9-137.4 kPa) and the second distillation column is operated at the same low pressure or at a pressure moderately higher (137.4-824.5 kPa higher) than the first column. The process discussed herein calls for the operation of a first distillation column at a substantially higher pressure (825-2040 kPa), giving a product stream of dimethylether and methanol wherein the molar ratio of DME to methanol is much greater than unity. Such a stream containing a significant amount of dimethylether is excellent feedstock for a methanol-to-gasoline conversion unit.
It is an object of the present invention to provide a method and apparatus for reducing the water content of the feed to a conversion reactor in an MTG plant.
It is a further object of the present invention to provide a method and apparatus for increasing catalyst life in a conversion reactor and/or operating the conversion reactor at either a higher temperature or at a lower recycle gas ratio, with the benefits resultant therefrom.